I am writing a series of mini-novels based on a group of teenagers who all have different backgrounds and ethnicities. How should I introduce each character? I mean, they are all kind of main characters. Each of them is going to have a major issue in each book that will be dealt with. In a way, these are books to help teenagers. Are there similar books to mine that I can look to for reference? Is it best to have one main character that tells the story? What do you think I should do?
If you are providing character diversity in part to make it easier for a diverse readership to identify with at least one character, then you probably do not want one character as the narrator of the entire book. A reader naturally feels a greater emotional connection to such a narrator. Furthermore, such makes it more difficult to express the less externally observable traits of the characters (thoughts and feelings).
So, if that is your goal (as seems to be the case based on each character having an issue to deal with), you should probably use a third person narrator, different characters as narrators for different portions, or some combination. With the stated length ("mini-novel") and target audience, changing the narrator sufficiently often to give each significant character equal play may be difficult.
With respect to introducing multiple main characters, there are many options. The book might start out in a scene with most or all of these characters in a common situation (e.g., having lunch together at the school cafeteria, being in line to sign up for something [possibly with one or more characters behind the sign-up table, signing up for something else, or just hanging around in the area], or being addressed by the non-main character leader of a homecoming float-building group), very briefly describing each character. (Third person narrative works well in such a setting.) The action might then focus on one or two of the main characters with the focus changing over longer periods of time (with gradual or abrupt transitions). Or there might be interaction among all or most of the main characters right from the start--e.g., friendly conversation, intentional rudeness to exclude an outsider, some characters gossiping about others.
It is also possible to gradually introduce characters, starting with two or three in the first scene and linking each of these to the other characters. This would tend to fit well with using multiple narrators. The introduction order can be used to contrast the significance of the character or to emphasize the relative isolation of the characters introduced later. E.g., quiet beret girl, who is not introduced until half way through the book, might be key to half the characters understanding their issues or she might be a character of no special importance whose issue is loneliness or telling a friend something difficult (both play on the isolation--in the first case being descriptive of her current state, in the latter case the isolation is something that she fears may come if she tells her friend). If quiet beret girl has the former role, it may be appropriate to foreshadow that she is significant by giving her one or two cameos early in the story. For example, the following both introduces and excludes beret girl:
"Hey, Susan," he called from behind the table.
"Hi, Juan," she smiled, leaning to see past a girl in a grey beret.
When introducing characters, particularly for your audience, length, and character count, it is helpful to present early a few memorable points onto which the reader can latch. E.g., quiet beret girl, non-nerd with glasses guy, rough-and-tumble string bean guy, brainy fashionable girl, poor leader girl, totally average guy. Such helps the reader establish a base upon which to build an understanding of the character and more easily identify the character in later scenes.
Well, let's start from the initial premises: they are a group. Their stories will follow different paths and indeed the narrator for each could change to the character themselves, but maybe that is not the best idea. Because this way you risk to end up with, let's say five, different and unrelated stories of people that somehow make up a group somehow in a given moment.
To ease such transitions, you, the author, could take the role of narrator and expose 'the story of group x' comprising ... and give each character a presentation. Then make them interact, and let them tell their own stories by such interaction.
Think of the group as a character. Each person has his/her own reasons to belong in the group and information and stories flow freely when the group is taking a rest maybe, during group activities or something but mostly in action, when they do what they do, something comes up with each member of the group and there's reasons for that, and explanations, and an answer to the problem, and friendship and a solution, and the premise that the group works best when everyone co-operates and solves the problem of each member (ok, maybe not all at once or in a row, it kinda kills momentum but there's the general mechanics).
Make the characters tell their own stories; they have friends within that group, they talk, and they solve their problems/differences/find ways to make the group perform better as a whole. They can be a group of friends/colleagues at universities/schools, they could be co-workers or children in a holiday camp, or members of a military squad or a band/group of artists, etc. For each setting various flavors of problem/solution/interaction are possible so the premise is important in world-building. Target audience is important... the degree of complexity of interactions and subtext messages are also important.
Context, subtext and other messages
Speaking of teenage targeted media, I remember watching (as a kid) some Japanese cartoon series, can't remember the proper name now, anyway, typical giant robot, alien invasion stuff, with soldiers, heroes etc. Now I re-watched as an adult (don't ask why, Covid made me exhaust all books and movies and I turned to "occult readings" again) and what did strike me was the strong political subtext I ignored as a kid - the idea that they had a government that took aberrant decisions, that they had people die because of corruption of certain civil servants and that incompetence led to the crisis in the first place, both with humans as well as aliens, the idea that there were double agents, importance of diplomacy and presence of manipulative characters with hidden agendas etc. Such important and loaded message that simply passed over my head as a kid - but did imprint some basic ideas and showed me "how the world works" in real life, in the guise of a group of heroes fighting aliens with huge robots...
Possible method for group of character stories
So, think of this: you can write anything of the sort easily if you figure out first: who are the people in the group, what kind of a group is it, what do they do - and what problems do each of them have. Then think of the target audience: will the message be passed directly? Is there a smoker that needs to quit smoking, or are we living in a magical world where certain spells are akin to drugs and make people happy for a short while but sick for a long time etc.
And then think of the general tone. Will it be a happy-go-lucky comedy where people have easy to solve and trivial problems that they do solve simply and everyone is happy, is it realist with real world serious problems or is it grim dark when everyone is basically more or less doomed but doomed nonetheless?
And then set the moral scheme for the story. The working idea- cooperation, problem solving, group efficiency that increases as all cooperate,friendship (is magic) or by contrary, tragic, hard thought provoking problems and everyone searching for solutions?
Then ... once you have the idea worked out, flesh it out with appropriate materials but don't skimp on ideas. Are we going to have both positive and negative examples? Will any character fail to do what they proposed to? Would there also be tragic characters, although the general vibe is good? What is the general message? Good luck!
The "Power of Five" series by Anthony Horowitz tells the story of a group of five characters, and each book has one other of these characters as the protagonist (and tells the story from his or her perspective). Sharon Shinn in her "Twelve Houses" series does the same.
Both start the first book like a normal novel about one character. In the course of the story the other characters are introduced (the first protagonist meets them). Each book tells a complete adventure and can be read on its own. The protagonist of that adventure is the protagonist of the book, the other group members are supporting characters. At the same time, a larger story arc runs in the background. This background story culminates in the last volume. The whole group are the heroes of that background story, and each individual's adventure is a key stone in the large storyline.
Both these series are constructed well, I can recommend them as good examples for this type of storytelling. (They have other weaknesses that are not relevant to your question. The best individual volumes from a pure reader perspective in my opinion are the first by Horowitz, "Raven's Gate", and a post-series supplemental volume by Shinn, "Fortune and Fate".)